The present invention relates to a method for injecting fuel into the combustion chamber of a direct-injection internal-combustion engine, notably of Diesel type, comprising a fuel injection nozzle allowing to obtain a high engine injection sensitivity.
The invention also relates to an internal-combustion engine using such a method.
As it is generally admitted by specialists, comparison of engines with different capacities and injection systems is generally made by means of the following formulation allowing to determine the injection sensitivity (in mm3/30 s) of the engine:   Sensitivity  =      Permeability    xc3x97          50      Power        xc3x97                            P_inj          -          50                    1300        xc3x97          0.5      L_cyl      
where Permeability represents the permeability of the injection nozzle (in mm3/30 s), Power the power per liter of the engine (in kW/l), L_cyl the capacity of the engine (in liters) and P_inj the maximum pressure of the injection system (in bar).
Selection of this sensitivity results from a compromise between the performance requirements at full load (torque and power) and the pollution requirements at partial loads.
An engine having a sensitivity value below 380 mm3/30 s is thus generally used to take account of the constraints linked with the pollution regulations and to provide sufficient power.
By way of illustration, FIG. 1 shows the evolution of the power (P) of the engine (in kW/l), laid off as ordinate, as a function of the injection sensitivity (S) of this engine (in mm3/30 s), laid off as abscissa.
It can be seen that, up to point C, the engine power increases substantially linearly with the injection sensitivity increase, then, from point C and up to point D, the power increase of the engine is minimal whereas the injection sensitivity increases significantly and, from point D, the sensitivity increases whereas the engine power decreases.
At partial loads, as it has been proved on many occasions, an increase in the injection sensitivity leads to an increase in emissions (NOx, particles and CO).
As regards pollutants, again by way of example, FIG. 2 shows the evolution of emissions (E) in g/kWh (NOx/10 or particles), laid off as ordinate, as a function of the injection sensitivity (S), laid off as abscissa, on a working point representative of urban driving conditions.
It can be observed that, for an injection sensitivity increasing by about 40 mm3/30 s from point A to point B, an emissions increase of the order of 0.15 g/kW is obtained and, beyond this point B, emissions increase very significantly.
FIG. 3 shows the evolution of the injection sensitivity on a graph where the emissions (E) at partial loads (NOx/10 or particles) are laid off as abscissa and the power (P) of the engine as ordinate. It can be seen that, to comply with the pollution regulations, the injection sensitivity has to be maintained below 380 mm3/30 s, which corresponds to point G in this graph.
In order to overcome the aforementioned drawbacks and notably the compromise between anti-pollution regulations and engine performance (power and torque), the applicant has developed a fuel injection method allowing to obtain a substantial engine power increase while significantly decreasing emissions.
This engine runs in two combustion modes. A conventional Diesel type combustion mode, with fuel injection around the combustion top dead center and diffusion combustion, which is preferably used at high loads.
By changing the injection strategy, the engine runs in another combustion mode, referred to as homogeneous mode, which is used at low loads.
The invention thus relates to a method for injecting fuel into the combustion chamber of a direct-injection internal-combustion engine, said chamber being delimited by the wall of a cylinder, a cylinder head and a piston comprising a bowl inside which a teat is arranged, characterized in that the fuel is injected by means of an injection nozzle allowing to obtain an engine injection sensitivity greater than or equal to 380 mm3/30 s and having a nappe angle less than or equal to       2    ⁢    Arctg    ⁢          CD              2        ⁢        F              ,
where CD is the diameter of the cylinder and F is the distance between the point of origin of the fuel jets and the position of the piston corresponding to a crankshaft angle of 50xc2x0 in relation to the top dead center (TDC).
Advantageously, the sensitivity can range between 380 and 520 mm3/30 s.
The fuel can be injected with a fuel jet nappe angle less than or equal to 120xc2x0.
The fuel can be injected with a nappe angle ranging between 40xc2x0 and 100xc2x0.
The invention also relates to an internal-combustion engine comprising at least a cylinder, a cylinder head, a piston sliding in this cylinder, a fuel injection nozzle and a combustion chamber delimited on one side by the upper face of the piston comprising a teat pointing towards the cylinder head and arranged in a bowl, characterized in that this engine comprises a fuel injection nozzle allowing to obtain an injection sensitivity greater than or equal to 380 mm3/30 s and having a nappe angle less than or equal to       2    ⁢    Arctg    ⁢          CD              2        ⁢        F              ,
where CD is the diameter of the cylinder and F is the distance between the point of origin of the fuel jets from the injection nozzle and the position of the piston corresponding to a crankshaft angle of 50xc2x0 in relation to the top dead center (TDC).
The nappe angle of the injection nozzle can be selected between 0xc2x0 and 120xc2x0, preferably between 40xc2x0 and 100xc2x0.
The angle at the vertex of the teat can be selected greater than the nappe angle by a value ranging between 0xc2x0 and 30xc2x0.
The axes of the fuel jets can form an angle of intersection of the order of 5xc2x0 with the flank of the teat.
The bowl can have an inclined lateral wall and the angle of inclination of this wall is less than 45xc2x0.